Transmission devices or automatic variable-speed transmissions known from practice are usually built with hydraulically actuated frictional shifting elements for obtaining various gear ratios. To be able to operate such transmission devices with an efficiency as great as possible, particular frictional shifting elements are also replaced by claw shifting elements which, compared with frictional shifting elements, are characterized by lower drag torques. During the operation of a transmission device the actuating pistons of the frictional shifting elements or claw shifting elements are preferably actuated hydraulically on one side in the closing direction and acted upon by a restoring spring mechanism in the opening direction with a spring force, so that when the actuating pressure is low enough they are shifted out of a force flow of the transmission device and changed to an open operating condition.
Moreover, from practice shifting elements are also known, which in the area of their actuating pistons are acted upon by hydraulic actuating pressures in both the closing direction and the opening direction, and then a restoring spring mechanism that acts in the direction of a shifting element is no longer necessarily needed. Depending on the application concerned, where appropriate restoring spring mechanisms with small spring constants can be provided for holding the actuating piston in the starting position equivalent to an open operating state of the shifting element.
As is known, the additional hydraulic restoration of such shifting elements requires in each case an actuating piston or double-piston in the form of a differential piston. Here, a first functional surface of the actuating piston, on which the actuating pressure that acts in the closing direction of the shifting element is applied, is larger than a second functional surface of the actuating piston on which an actuating pressure that acts in the opening direction of the shifting element can be applied. If such a differential piston is acted upon simultaneously in the area of the first functional surface and in the area of the second functional surface by equal pressures, then by comparison with one-sided pressurization of the actuating piston a lower piston force level is produced.
The above-described piston force level can be appropriately adjusted to improve the quality of shift operations in which a support element in the area of the shifting element is substantially smaller than in at least one other shift operation by means of which a gear ratio in a transmission device with a considerably larger support element is to be engaged. This means that at least one gear ratio with a high support element in the area of the shifting element is produced by an actuating pressure in the area of the second functional surface of the actuating piston, which is essentially equal to zero. Gear ratios from which smaller support elements at the shifting element result, are obtained with an actuating pressure applied in the area of the second functional surface.
Disadvantageously, for the hydraulic restoration of an actuating piston of a shifting element in each case an additional pressure oil supply has to be provided, and this entails undesirably high design complexity and cost, particularly in the case of shifting elements in the form of clutches. Furthermore, an electro-hydraulic transmission control system for providing the actuating pressure that must be applied to the second functional surface of the actuating piston has to be designed and built with additional control elements such as valve mechanisms and the like, whereby in turn the units takes up more structural space and the manufacturing costs of the transmission device are increased.
From DE 197 00 635 C2 a clutch arrangement for an automatic transmission is known, in which an actuating piston of a frictional shifting element can be acted upon in the area of a first functional surface with an actuating pressure acting in the closing direction of the shifting element. In the area of a second functional surface, the actuating piston of the shifting element can be acted upon by an actuating pressure of a second, also frictional shifting element, such that a piston space of the further shifting element, in the area of which the further shifting element can be acted upon by the actuating pressure that acts in the closing direction of the further shifting element, is delimited by the actuating piston of the shifting element and the actuating piston of the further shifting element.
In the clutch arrangement an above-described hydraulic restoration of a shifting element by acting upon the actuating piston of the shifting element in the area of its second functional surface, i.e. in the opening direction of the shifting element, or the production of a reduced piston force level acting on the actuating piston of the shifting element, can only be accomplished with substantial control and regulation complexity.